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Show 1896.] METALLIC COLOURS OF BIRDS. 293 in the wing-quills iu the direction of diminished efficiency for flight would be checked by natural selection. So far w e have seen h o w the metallic colours both of Sun-birds and Humming-birds depend in each case upon a combination of a certain structure and a black pigment. In conclusion something may be said as to the colours themselves. In the Sun-birds a greenish-blue seems to be the most primitive metallic colour, and this is a very common tint elsewhere, e. g. Peacock, Quezal, &c. According to Gadow, one of the reasons why any metallic feather does not display all the colours of the spectrum is probably because the overlapping of successive colour-producing structures cuts out certain of the rays. If this overlapping really occurs it seems not unnatural to conclude that the middle rays of the spectrum, those in the neighbourhood of the green, would be least likely to be affected, and w e would thus get green as a primitive metallic colour. The combination of this structure with a surface sculpturing might produce a purple or violet tint; the absence of red and yellow may not improbably be a result of physical conditions. Walter explains the rarity of red and yellow metallic colours as due to the nature of the pigments contained in the coloured tissues, but this again is difficult to harmonize with our knowledge of such pigments. There can be little doubt that in Humming-birds a greenish-yellow is the most primitive metallic tint. It is suggested even in the " hermit " forms, and is very widely spread on contour-feathers elsewhere. In the absence, however, of any suggested physical explanation of the metallic colours of Humming-birds, it would perhaps be premature to attempt to account for the wronderful range of colour found in the family. As to the distribution of metallic colour one or two facts still remain to be noticed. Thus metallic colour is not always characteristic of the male. In Eustephanus fernandensis female metallic colour is more or less distinctly present over nearly the whole of the upper and under surfaces, the tail-quills show bright metallic colour, and the head bears a special metallic crest. The male, on the other hand, has no metallic colour except the bright crest, the rest of the body is cinnamon-coloured and without metallic gloss. The absence of metallic colour is apparently to be accounted for here by the absence of the usual blackish-brown pigment. Again, a specimen marked Topaza pella, young male, which was examined, showed metallic-greenish feathers in the upper part of the head, a spot which in the adult is covered with black feathers. Similarly, Salvin notices that in Lampomis mango the throat in the young bird is covered with glittering green feathers, and in the adult with pure black ones. Thus apparently an excess of black pigment is as fatal to the display of metallic colour as its total absence. As to the relation between a black colour and metallic tints there are some other interesting facts. In Cyanolesbia gorgo the tail-quills are greatly elongated and show gorgeous metallic colour, but this is confined to the distal end of the feathers, the proximal region being a |